To date, directional couplers have been used for, for example, measurement of high-frequency signals. See, for example, Japanese Unexamined Patent Application Publication No. 2009-044303 (Patent Document 1).
FIG. 1(A) is a block diagram of an RF transmission circuit 100 used in, for example, cellular phones. The RF transmission circuit 100 includes an antenna 111, a directional coupler 120A, a transmission power amplifier 113, a modulation circuit 112, and an automatic gain control circuit 114. The directional coupler 120A, which is of a transmission line type, includes a main line 121 and a coupling line (sub line) 122. The main line 121 is connected between the antenna 111 and the transmission power amplifier 113. The automatic gain control circuit 114 is connected to the directional coupler 120A and the sub line 122, and controls the transmission power amplifier 113 on the basis of a signal from the sub line 122 which is coupled to the main line 121.
FIG. 1(B) is an equivalent circuit diagram of the directional coupler 120A. Here, the directional coupler 120A is assumed to be an ideal circuit, in which the coupling factor of a mutual inductance M between the main line 121 and the sub line 122 is 1. The main line 121 has a signal input port RFin and a signal output port RFout, and the sub line 122 has a coupling port CPL and an isolation port ISO. The main line 121 and the sub line 122 are coupled to each other through electric field coupling due to distributed capacitances C between the two lines, and at the same time coupled to each other through magnetic field coupling due to the mutual inductance M.
When a signal S1 is input from the signal input port RFin in the main line 121, a signal S2 propagates toward the coupling port CPL and a signal S3 propagates toward the isolation port ISO, in the sub line 122, due to electric field coupling caused by coupling capacitances C. A signal S4 and a signal S5 propagate in a direction from the isolation port ISO to the coupling port CPL in a closed loop formed of the sub line 122 and the ground (GND), due to magnetic field coupling caused by the mutual inductance M.
In this ideal equivalent circuit, the signal S2 and the signal S4 that flow to the coupling port CPL both have a phase of +90° with respect to the signal S1, i.e., the same phase. Hence, a signal having a power which is the sum of the power of the signal S2 and the power of the signal S4 is output from the coupling port CPL. On the other hand, regarding the signals S3 and S5 that flow to and from the isolation port ISO, the signal S3 has a phase of +90° with respect to the signal S1, and the signal S5 has a phase of −90° with respect to the signal S1, that is, the signal S3 and the signal S5 have opposite phases. Hence, the power of the signal S3 and the power of the signal S5 cancel each other out, whereby no signals are output.
FIGS. 2(A) and 2(B) are diagrams illustrating the frequency characteristics and isolation characteristics of the directional coupler 120A. Referring to the frequency characteristics illustrated in FIG. 2(A), the insertion loss is approximately zero over the whole frequency range, and the amount of isolation of the isolation port ISO is extremely small compared with the amount of coupling of the coupling port CPL. Hence, a high directivity is obtained. The isolation characteristics illustrated in FIG. 2(B) illustrate, using polar coordinates, a signal output from the isolation port ISO, which is always approximately zero irrespective of the frequency.